feat(books): ajouter le champ format en base et l'exposer dans l'API

- Migration 0020 : colonne format sur books, backfill depuis book_files
- batch.rs / scanner.rs : l'indexer écrit le format dans books
- books.rs : format dans BookItem + filtre ?format= dans list_books
- perf_pages.sh : benchmarks par format CBZ/CBR/PDF

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

infra/migrations/0020_add_format_to_books.sql

@@ -0,0 +1,13 @@
+-- Add format column to books table (denormalized from book_files for easy API access)
+ALTER TABLE books ADD COLUMN IF NOT EXISTS format TEXT CHECK (format IN ('pdf', 'cbz', 'cbr'));
+
+-- Backfill from book_files (take the format of the most recent file per book)
+UPDATE books b
+SET format = bf.format
+FROM (
+    SELECT DISTINCT ON (book_id) book_id, format
+    FROM book_files
+    ORDER BY book_id, updated_at DESC
+) bf
+WHERE b.id = bf.book_id
+  AND b.format IS NULL;